JP2001206785A - Method of producing silicon carbide porous body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a silicon carbide porous body having high porosity and sufficient mechanical strength. SOLUTION: The method of producing a silicon carbide porous body comprises heat treating a formed body containing 70 to 85 mass % silicon carbide particles having an average diameter of 0.2 to 3 μm and 15 to 25 mass % inorganic hollow particles having an average diameter of 30 to 60 μm, with the proviso that the total amount of the silicon carbide particles and the inorganic hollow particles is >=95 mass %, under a non-oxidizing atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon carbide porous body having high porosity and excellent mechanical strength.

[0002]

2. Description of the Related Art Silicon carbide has excellent properties as a heat-resistant material such as low thermal expansion coefficient and high mechanical strength at high temperature, in addition to high hardness, excellent wear resistance and excellent chemical stability. I have. For this reason, attempts have been made to apply the silicon carbide porous body as a high-temperature filter for separating ash and the like contained in high-temperature combustion gas, such as a filter for a dust removal device of a diesel engine.

The silicon carbide porous material used as a filter has not only heat resistance and corrosion resistance, but also low fluid permeation resistance and high-efficiency removal of foreign particles.
A pore diameter and a porosity according to the target foreign particles are required.

Generally, when the porosity or the pore diameter is increased, the mechanical strength of the porous body tends to decrease. For example, as a method for producing a silicon carbide-based porous body, silicon carbide particles having a relatively large particle diameter of several μm or more are used as an aggregate, and a binder such as glassy or clay is added thereto, followed by molding. Methods of baking a body at a temperature at which the binder melts are conventionally known. However, in such a conventional method, it is relatively easy to obtain a pore diameter of 1 μm or more, but since coarse silicon carbide particles are used as the aggregate, the mechanical strength is low and the porosity is low. Is 3
In the case of a porous body of about 0 to 35%, the mechanical strength is 20 to 3
Usually, it was about 0 MPa.

Further, a method for producing a silicon carbide porous body using glass beads having an average particle diameter of 45 μm is proposed in “Powder and Powder Metallurgy”, Vol. 43, No. 12, page 1461. However, a silicon carbide-based porous body having a well-balanced porosity, pore diameter, and mechanical strength has not been obtained.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a silicon carbide porous body having a large pore diameter, a high porosity, and a sufficient mechanical strength.

[0007]

The present invention comprises 70 to 85% by mass of silicon carbide particles having an average particle diameter of 0.2 to 3 μm and 15 to 25% by mass of inorganic hollow particles having an average particle diameter of 30 to 60 μm, The present invention also provides a method for producing a silicon carbide porous body, which is formed into a porous body by heat-treating a molded body having a total amount of the silicon carbide particles and the inorganic hollow particles of 95% by mass or more in a non-oxidizing atmosphere.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a silicon carbide based porous material of the present invention (hereinafter referred to as the present production method), 70 to 85% by mass of silicon carbide particles having an average particle diameter of 0.2 to 3 μm and an average particle diameter of 30 A molded article containing 15 to 25% by mass of inorganic hollow particles of about 60 μm and having a total amount of the silicon carbide particles and the inorganic hollow particles of 95% by mass or more is used.

In the present invention, the inorganic hollow particles include:
Any material that forms pores during the heat treatment and acts as a sintering aid for the silicon carbide particles during the heat treatment is preferably used.

It is preferred that the inorganic hollow particles are glassy hollow particles mainly composed of an Al ₂ O ₃ component and a SiO ₂ component, because sintering is promoted and the mechanical strength of the porous body is increased. More preferably, the content of the Al ₂ O ₃ component in the inorganic hollow particles is 20 to 80% by mass, and the content of the SiO ₂ component is 80 to 20% by mass.

The Al ₂ O ₃ component in the inorganic hollow particles reacts with a metal oxide such as silica present as a trace impurity in the silicon carbide particles to form a liquid phase and contributes to strengthening the bond between the silicon carbide particles. In addition, some react with the silicon carbide particles to generate gas components such as AlO and Al ₂ O and volatilize out of the system. At this time, it is considered that this contributes to the formation of open pores connected to the outside. The SiO ₂ component in the inorganic hollow particles is
It is considered that it reacts with silicon carbide to generate gas components such as CO and SiO, and promotes formation of interconnected pores like the Al ₂ O ₃ component.

In the present invention, the inorganic particles need to be hollow. If the inorganic particles are not hollow, it is difficult to remain as pores in the porous body after reacting with the silicon carbide particles. As long as the inorganic particles are hollow, the portion corresponding to the outer skin may be dense or porous. The external shape of the inorganic hollow particles is preferably spherical because it is easily available, but may be a shape other than spherical.

The content of the inorganic hollow particles is 15 to 25% by mass in the molded product. If the content is less than 15% by mass, the amount of the inorganic hollow particles that react with the silicon carbide particles during the heat treatment is small, so that the porosity is as low as about 25%. On the other hand, if the content exceeds 25% by mass, a porous body having a porosity of more than 60% can be obtained, but on the other hand, the pore diameter formed in the porous body becomes too large, resulting in a decrease in mechanical strength.

In the present invention, the average particle diameter of the inorganic hollow particles is 30 to 60 μm. When the average particle diameter of the inorganic hollow particles is less than 30 μm, the mechanical strength of the porous body is improved, but the pore diameter of the porous body becomes too small. On the other hand, when the average particle diameter of the inorganic hollow particles exceeds 60 μm, the porosity and the average pore diameter can be increased, but giant pores are easily generated, resulting in a decrease in mechanical strength.

When the inorganic hollow particles have a silicon carbide layer on the surface, even if the content of the inorganic hollow particles increases, the pore diameter of the porous body is prevented from becoming too large,
As a result, a decrease in mechanical strength can be prevented, which is preferable.
The method for producing the inorganic hollow particles having a silicon carbide layer on the surface is not particularly limited, but a wet method is convenient and preferred.

In the case of the wet method, the inorganic hollow particles float on the surface in a normal liquid. Therefore, simply adding the inorganic hollow particles into a slurry obtained by dispersing silicon carbide particles in water uniformly coats the surface. Can not. Therefore, it is preferable to adjust the pH of the slurry to a range of 6 to 8. In a slurry having a pH of 6 to 8 in which silicon carbide particles are dispersed, uncoated inorganic hollow particles are generally positively charged, but silicon carbide particles are negatively charged. The particles adhere electrostatically and are easily coated.

Further, by pouring the slurry onto a sieve having an opening smaller than the diameter of the hollow sphere, inorganic hollow particles having a silicon carbide layer on the surface (hereinafter referred to as silicon carbide coated particles) can be recovered from the slurry. . After drying the silicon carbide-coated particles, a heat treatment is performed at a temperature (for example, 500 to 800 ° C.) at which the inorganic hollow particles do not soften, whereby the adhesion of the silicon carbide particles to the surface can be further strengthened. This operation is sufficient even once, but it is possible to coat more uniformly by repeating the same operation several times.

In the present invention, the average particle diameter of the silicon carbide particles is 0.2 to 3 μm. When the average particle diameter of the silicon carbide particles is less than 0.2 μm, the handleability of the raw material powder is reduced, the mixing with the inorganic hollow particles becomes insufficient, and defects in the porous body are likely to occur. When the average particle diameter of the silicon carbide particles exceeds 3 μm, the sinterability is reduced, and it becomes difficult to obtain sufficient mechanical strength. The type and purity of the silicon carbide particles are appropriately selected depending on the purpose and application.

In the present invention, the total amount of the inorganic hollow particles and the silicon carbide particles is 95% by mass or more in the molded product. If the total amount of the inorganic hollow particles and the silicon carbide particles is less than 95% by mass in the molded body, a porous body having desired characteristics cannot be obtained. In the present invention, the inorganic hollow particles and the silicon carbide particles are preferably mixed by a general mixing means such as a ball mill or a mixer.

As a method for producing a molded article containing inorganic hollow particles and silicon carbide particles, a usual ceramic molding method such as press molding or extrusion molding is appropriately employed. Note that an organic binder may be added during molding. Examples of such an organic binder include polyvinyl alcohol or a modified product thereof, starch or a modified product thereof, carboxymethylcellulose, hydroxylmethylcellulose, polyvinylpyrrolidone, an acrylic resin or an acrylic copolymer, a vinyl acetate resin or a vinyl acetate copolymer, And other organic substances can be used.

The heat treatment of the compact is preferably carried out at 1900 to 2000 ° C. for 1 to 10 hours in a non-oxidizing atmosphere. If the temperature is lower than 1900 ° C., the mechanical strength of the porous body may be insufficient due to weak bonding between silicon carbide particles, and if it exceeds 2000 ° C., silicon carbide is decomposed, which is not preferable.

If the temperature holding time is less than 1 hour, the reaction between the silicon carbide particles and the inorganic hollow particles does not proceed sufficiently, which is not preferable. If the temperature holding time is more than 10 hours, the decomposition of silicon carbide proceeds and the pore size increases. In addition, the porosity becomes too large and the mechanical strength of the porous body is reduced, which is not preferable.

The rate of temperature rise during the heat treatment depends on the size of the compact,
It is appropriately selected depending on the shape and the like, but 100 to 400 ° C./h
Is preferable because the inside of the compact can be heated almost uniformly. If the temperature is in the range of 1900 to 2000 ° C. even during the heating process, the elapsed time is added to the holding time. Here, the non-oxidizing atmosphere refers to an atmosphere containing substantially no oxygen, and an inert atmosphere such as argon or helium is preferably employed.

The porosity of the porous silicon carbide obtained by the present invention is preferably 35 to 60%. If the porosity is less than 35%, the air permeability of the porous body is impaired, which is not preferable. If the porosity exceeds 60%, the mechanical strength of the porous body is reduced, which is not preferable.

The average pore diameter of the silicon carbide porous body obtained by the present invention measured by a mercury porosimetry is 5 to 15 μm.
m is preferable. If the average pore diameter is less than 5 μm, the air permeability of the porous body is impaired, which is not preferable. If the average pore diameter exceeds 15 μm, the mechanical strength of the porous body is reduced, which is not preferable.

The silicon carbide porous body obtained by the present invention has a four-point bending strength of 100 in accordance with JIS R1601.
When the pressure is at least MPa, the porous body has sufficient mechanical strength even if the wall thickness of the porous body is reduced, so that the filter can be reduced in weight and size, which is preferable. The silicon carbide porous body obtained by the present invention preferably contains silicon carbide in an amount of 80% by mass or more.

[0027]

Examples Examples of the present invention (Examples 1 to 4) and comparative examples (Examples 5 to 10) are shown below. The porosity and bulk density of the porous body were measured by the Archimedes method.
It was measured in accordance with JIS R1601. Pore characteristics are determined by mercury porosimeter (AUTOS manufactured by Yuasa Ionics, Inc.)
CAN-33). Table 1 shows the measurement results of each example.

Example 1 100 g of silicon carbide particles (A-1 manufactured by Showa Denko KK) having an average particle diameter of about 0.6 μm were put into 1 L of ion-exchanged water, the pH was adjusted to 7, and the mixture was stirred with a stirrer. While dispersing ultrasonically, a slurry of silicon carbide particles was prepared. Spherical glass hollow particles having an average particle diameter of 45 μm (MIC manufactured by Chichibu Onoda Co., Ltd.)
ROCELS SL75), silicon carbide particles were adhered to the spherical hollow particles while stirring with a stirrer, and then passed through a sieve having an opening of 36 μm, and the spherical hollow particles having silicon carbide particles adhered on the sieve were removed. Collected. The collected spherical hollow particles were heat-treated in the atmosphere at 500 ° C. for 5 hours to obtain silicon carbide-coated particles.

Next, 15 parts by mass of the silicon carbide-coated particles,
85 parts by mass of the same silicon carbide particles as those used for coating were dry-mixed with a mixer to obtain mixed particles. After the mixed particles were formed into a size of 40 mm × 70 mm × 5 mm by a mechanical press under a pressure of 20.3 MPa, the formed body was placed in a graphite container and heat-treated using a resistance heating furnace. The heat treatment was performed under an argon atmosphere at a heating rate of 40.
The temperature was raised to 1970 ° C. at 0 ° C./h, and kept at 1970 ° C. for 2 hours.

Example 2 In Example 1, the amount of the silicon carbide-coated particles was changed from 15 parts by mass to 20 parts by mass, and the amount of the silicon carbide particles was changed from 85 parts by mass to 80 parts by mass.
Same as Example 1.

Example 3 The procedure of Example 2 was repeated except that the silicon carbide-coated particles were replaced with glass hollow spherical particles not coated with silicon carbide.

Example 4 Except that the amount of silicon carbide-coated particles was changed from 15 parts by mass to 25 parts by mass and the amount of silicon carbide particles was changed from 85 parts by mass to 75 parts by mass in Example 1,
Same as Example 1.

Example 5 In Example 1, the amount of the silicon carbide-coated particles was changed from 15 parts by mass to 30 parts by mass, and the amount of the silicon carbide particles was changed from 85 parts by mass to 70 parts by mass.
Same as Example 1.

Example 6 Except that the amount of silicon carbide-coated particles was changed from 15 parts by mass to 10 parts by mass and the amount of silicon carbide particles was changed from 85 parts by mass to 90 parts by mass in Example 1,
Same as Example 1.

Example 7 The procedure of Example 6 was repeated, except that the silicon carbide-coated particles were replaced with glass hollow spherical particles not coated with silicon carbide.

Example 8 The procedure of Example 1 was repeated except that the average particle diameter of silicon carbide particles was changed from 0.6 μm to 5 μm.

Example 9 The procedure of Example 1 was repeated, except that the hollow glass spherical particles in Example 1 were changed from those having an average particle diameter of 45 μm to 20 μm.

Example 10 The procedure of Example 1 was repeated, except that the hollow glass spherical particles were changed from those having an average particle diameter of 45 μm to those having a mean particle diameter of 100 μm.

[0039]

[Table 1]

[0040]

The porous silicon carbide obtained by the production method of the present invention has a high porosity, a high mechanical strength, and a pore size distribution suitable for removing and separating relatively coarse particles.
When used in a high-temperature dust collection filter or the like, dust can be removed without causing a remarkable reduction in filtration speed, and sufficient durability is provided.

Claims (4)

[Claims] 1. The method according to claim 1, comprising 70 to 85% by weight of silicon carbide particles having an average particle diameter of 0.2 to 3 μm and 15 to 25% by weight of inorganic hollow particles having an average particle diameter of 30 to 60 μm. A method for producing a silicon carbide porous body, wherein a molded body having a total amount of hollow particles of 95% by mass or more is heat-treated in a non-oxidizing atmosphere to be a porous body. 2. The method according to claim 1, wherein said inorganic hollow particles are glassy hollow particles mainly composed of an Al ₂ O ₃ component and a SiO ₂ component. 3. The method according to claim 1, wherein the inorganic hollow particles have a silicon carbide layer on the surface. 4. The porosity of the porous silicon carbide body is 35 to 35.
60%, average pore diameter of 5 to 15 μm.
4. The method for producing a silicon carbide porous body according to 2 or 3.